The invention relates to pulping processes based on organic acids and particularly to the recovery of organic acids from bleaching liquors, such as alkaline filtrates and washing filtrates, obtained from bleaching or from stages closely associated with bleaching. At the same time, the process of the invention produces a valuable fertilizer by-product. The invention further relates to recycling of bleaching liquors. The invention is applicable in pulping processes based on organic acids, especially formic acid and/or acetic acid, wherein herbaceous plants or trees are used as raw material. The invention is also applicable in pulping processes employing other solvents and reagents.
Organic acids are produced in all pulping processes, but their utilization remains unsolved. In conventional processes, organic acids are usually harmful and consume the alkali used in the processes. Organic acids are formed not only in cooking, evaporation and drying of lignin, but also in bleaching. In alkaline hydrogen peroxide and/or oxygen/hydrogen peroxide bleaching, formic acid, acetic acid, oxalic acid and glycolic acid are typically produced. These acids react in the bleaching with alkalis to corresponding alkali metal salts. They add to the load of organic matter (COD) in waste waters and to the use of chemicals in water purification plants. The same problem appears in closed bleaching, because the salts do not evaporate, but remain in the concentrate. The treatment of the concentrate therefore continues to be an environmental and waste water issue.
FI patent publications 103 588 (WO 96/35013) and 103 899 (WO 98/20198) disclose processes based on formic acid which make use of the acids formed in cooking, evaporation and drying of lignin, the organic acids thereby formed being advantageous in view of process economy because they save cooking chemicals. Organic acids are formed as a result of hydrolysis reactions of hemicellulose and lignin and from acetyl groups of plants. In highly acid concentrations, esters are also produced. Esters may be converted back to organic acids, as described in FI patent application 973474 (WO 99/10595). However, these processes do not take advantage of the organic acids formed in the bleaching; on the contrary, the acids are lost in the alkaline bleaching, because they react immediately with the sodium hydroxide used in the bleaching, whereby salts are formed. Such salts are undesirable also in conventional processes because they get into waste waters.